Many design applications, particularly those involving battery charging and LED lighting, require power converters that can deliver precisely regulated load current. In most cases, the power converter is an isolating type, having a primary circuit and secondary circuit which are magnetically inductively coupled by an isolating power transformer. In almost all of these applications, there is a power controller situated on the primary side, with a current-sensing circuit on the secondary side, from which the feedback signal is derived and communicated to the primary-side controller using an optocoupler or similar means.
Resonant power converter topologies are well-suited to power conversion applications owing in large part to their virtues of high efficiency and low radio-frequency (RF) emissions. In particular, the half-bridge topology converters are becoming increasingly common, in particular those with LC (inductor-capacitor, also known as series-resonant), LLC (inductor-inductor-capacitor), LCC (inductor-capacitor-capacitor) and LLCC (inductor-inductor-capacitor-capacitor) topologies.
Many off-line power converter applications, such as LED drivers, require a constant current (CC) output characteristic which has very low ripple content and is precisely regulated. A common approach is to sense the output current directly on the secondary side and communicate this to the power controller on the primary side with an isolating device, such as an optocoupler. This method can increase the size and cost of a product. It is therefore advantageous to use a CC method which achieves a similar degree of precision at a lower cost.
Primary sensing current regulation (PSCR) is a method which can achieve an adequate degree of CC control in applications which have a relatively narrow range of line and load conditions. For example, single-stage fixed-output LED drivers using LC (series resonant) topology are already available which use PSCR to achieve a CC output characteristic which has good accuracy and low ripple. However, these examples are not capable of dimming the output accurately (by reducing the output current level) unless additional sensing and control circuitry are provided. To extend the dynamic range while retaining good efficiency and low RF emissions, it is often practical to change to either an LLC, LCC, or a LLCC topology. However, the primary and secondary currents in these topologies do not scale well across line and load conditions, rendering primary sensing current regulation impractical.
An example of an LLCC converter is shown in FIG. 6. In FIG. 7, the currents flowing through the various parts of the power converter are shown. FIG. 7b shows the current delivered from the transformer into the output rectifier block. FIG. 7f shows the current flowing through the primary current sense resistor R1, which is clearly very different, while FIG. 7g shows the error current. The current contributions of the parallel-resonant tank components C3, L3 change both the shape and the value of the sensed average current, creating errors that increase as the output power of a converter is reduced.
For example, US20130094248A1 (see FIG. 1) discloses a method of achieving primary-side current regulation which works by sensing the primary current. This method however is subject to the errors created by currents flowing through the reactive components which appear in the circuits described.
U.S. Pat. No. 7,948,774B2 (FIG. 2) and U.S. Pat. No. 8,842,449 B1 (FIG. 3) disclose a method of achieving primary-side regulation which works by sensing the voltage developed across a capacitor placed in series with the primary transformer winding. This method will regulate for average current provided that the frequency remains constant. However, this can only be achieved if the line and load conditions remain constant, or if the power delivery can be regulated without change in frequency, as in a phase-shifted full-bridge converter, for example. Again, this method is subject to the same estimation errors as described above.
US20150124489 A1 (FIG. 4) discloses a method of achieving primary-side regulation which works by sensing the voltage developed across an isolating current transformer placed in series with the primary transformer winding. This method will achieve some measure of regulation, provided that the line and load conditions remain constant, or if the power delivery can be regulated without change in frequency, as in a phase-shifted full-bridge converter, for example. Once again, this method is subject to the same estimation errors as described above.
US20140361698 (FIG. 5) discloses a method of achieving primary-side regulation which works by sensing the peak voltage developed across a resistor placed in series with one of the switching transistors of the bridge. This method will provide poor regulation unless the line and load conditions remain constant, or if the power delivery can be regulated without change in frequency, as in a phase-shifted full-bridge converter, for example. Yet again, this method is subject to the same estimation errors as described above.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.